A famous feature of nijoujou, the Tokugawa shoguns' castle in Kyoto, Japan. The wooden floorboards of the walkways surrounding all the rooms (traditional Japanese, tatami-covered) are rigged so that they rub against little metal pegs when people walk over them. This produces a slight squeaking sound. When several people walk over them at the same time, it really does sound a lot like a singing bird.

The point of all this was to defend against assassins by making it hard or impossible to sneak about soundlessly in the castle.

A rather clever device from Japanese architecture. Basically, a floor built so that it 'sings' when walked across. This was intended to warn the building occupants that an intruder was up to some skullduggery.

An example photo can be found here:

http://www.johnharveyphoto.com/Japan/Kyoto%20Day%201/NightingGaleFloors.html

Sources:

http://www.kansai.gr.jp/culture/build/archi_e.htm - This site has some sound files of what the floors sound like.

http://www.probertencyclopaedia.com/F5C.HTM